Many antilock braking systems derive an estimate of vehicle speed from the rotation of the vehicle wheels (called wheel speed) and another estimate of the vehicle speed by some means independent of wheel speed. A sufficient difference between these two estimates is indicative of wheel sliding on the road surface and calls for reduction in the braking pressure. There are many different schemes for deriving the vehicle estimate including free running fifth wheels, predictive schemes based on the wheel speed, and the provision of one of many types of accelerometers and then integrating the acceleration measurement to get an estimate of velocity. One common method is to provide one or more mercury switches for measuring acceleration (or its negative, deceleration). These mercury switch techniques seem to be adequate on conventional passenger cars yet inadequate on four wheel drive and similar vehicles. The schemes for predicting vehicle speed based on wheel speed can also lead to inaccurate results. For example, in a four wheel drive with locked differentials, if one wheel becomes locked, all will lock because of the rigid drive system, the system will have no reliable wheel speed information, and must predict (most likely inaccurately) velocity decrement until reliable information becomes available.
U.S. Pat. No. 3,677,094 utilizes a flexibly supported mass to measure vehicle acceleration and, by integration determined vehicle velocity. This patent teaches a technique for using the pitch of a vehicle relative to the inclination of the road on which the vehicle is moving to correct the measured vehicle velocity for road inclination.
Among the several objects of the present invention may be noted the provision of an improved accelerometer measurement for a moving vehicle; the provision of a new technique for estimating vehicle speed independent of wheel speed; the Provision of overall improvements in the operation of antilock braking systems for certain classes of vehicles; the Provision of a method of predicting vehicle velocity by measuring the compression of the front end suspension of the vehicle; and the provision of a simplistic device which measures the dip or forward rotation experienced by a vehicle when being braked and for translating that dip into an indication of the vehicle's velocity.
In vehicles with adaptive suspension systems as well as antilock braking systems, sensor information and/or control signals may be shared by both systems. Since there should be no dip with an adaptive suspension system, the information needed for the present velocity prediction might come from force, torsion or similar measurements.
Accordingly, a further object of the present invention is to provide a vehicle velocity measurement scheme which is compatible with adaptive suspension systems. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter.